1. Field of the Invention
This invention relates generally to platens for use in laminating presses and a method of laminating using such platens. In more particular aspects, this invention relates to an improved cooling system for platens for use in laminating presses and the use of such platens with improved cooling for lamination.
2. Background Art
In lamination operations, heat and pressure are applied to a stack of sheets of material often referred to as a book. The stack of sheets includes material which will join together under heat and pressure to form a unitary resulting product. In most lamination operations, it is necessary to control both the heating cycle and the cooling cycle to assure a proper fusing of the materials to insure a unitary finished product, and a proper cooling to minimize stresses induced in the resulting product. This is particularly important in the case of laminating materials together to form chip carriers or circuit boards. The cooling cycle is particularly important where flatness of the resulting product is required, the cooling cycle being important to minimize the induced stresses which could result in a product which becomes bowed or otherwise nonconforming to the flatness requirements. In addition, any in plane dimensional changes that occur during lamination must be linear and repeatable from day to day.
There have been several prior art proposals for platen designs which allow for cooling of the platen by passing cooling fluids through the platen. However, even these prior art proposals for cooling platens, in many instances, still permit unacceptable levels of resultant stresses after cooling.
There are three main types of lamination presses used in the fabrication of printed wiring boards (PWBs). They all use heated platens to apply heat and pressure to the product during the lamination process. Steam presses circulate fluid through the passages in the platen for both heating and cooling, and achieve very fast heat up rates and good uniformity, but are limited to peak temperatures in the range of 450xc2x0 F. Hot oil presses are configured in a similar manner and have similar heating characteristics, but are limited to peak temperatures in the range of 750xc2x0 F. The high heat capacity of the circulating fluid (steam or oil) allows for rapid heat transfer, even with small temperature differences between the fluid and the platen.
A third alternative is an electric press, which uses electric resistance cartridge heaters (installed in the platen for heating) and uses separate passages (like steam or hot oil passages) for circulating cooling fluids. Electric presses have several advantages, such as multi-zone control within the platen, much greater peak temperature capability, and diminished safety concerns (versus hot oil or steam). Configured with sufficient wattage heaters, very high heat up rates can be achieved and, with proper control systems, excellent temperature uniformity can be achieved. The typical installation uses up to three modes of cooling, depending on the current platen temperature: (1) air only; (2) air and water mist, and (3) water. However, uniform cooling of such presses can be problematic because the introduction of low temperature cooling fluids into the passages in the platen cools much faster at the inlet than the outlet. For example, when a press is operating at 600xc2x0 F. and cooling is required, room temperature air circulating in the passages will remove heat very quickly at the inlet, but almost no heat at all in the outlet.
Therefore, it is an object of the present invention to provide an improved platen and method of laminating using such platens, which reduces stresses induced during the cooling cycle, by improving the cooling of the workpiece following the heating cycle of the lamination operation.
According to the present invention, a platen for use in a laminating press is provided which includes a body of material having first and second faces and spaced first and second ends. Preferably, at least one heating device is disposed in the body of material. First and second spaced cooling channels are formed in the body of material, the first cooling channel being adjacent the first face and having a fluid inlet port adjacent to or in the first end, and a fluid outlet port adjacent to or in the second end, and a second cooling channel being adjacent the second face and having a fluid inlet port adjacent to or in said second end, and a fluid outlet port adjacent to or in the first end. The invention also contemplates using such platens for laminating a book or stack of sheets of material to form a unitary single member having reduced stresses.